Some factors, such as the antenna type and power output can be easily adjusted by an operator to manipulate the range of a radio signal. However, others (such as the weather) are out of operator control. What such factors can affect the range of a radio transmission, and how?
On HF, "range" is a function of propagation, which is affected by the signal's frequency, the solar flux, geogmagnetic conditions ("space weather"), the state of the ionosphere along the path the signal traverses, the number of "hops" required between reflective ionospheric layers and the earth, and whether the earth reflections are over land or saltwater.
Propagation prediction programs create models of the ionosphere and a signal's path through it to predict overall attenuation, which forecasts the fraction of the original signal's power that will be received at a particular destination.